The present invention relates generally to mechanical load limiters and, more particularly, load limiters for push-pull mechanical control linkages in aircraft.
Mechanical control linkages in airplanes are often long, relatively complex, and include multiple moving parts such as control rods, cables, and cranks. Because of weight load considerations, such mechanical linkages, including their associated structural support elements, are designed and built as light as possible. To this end, the various parts of the linkage are ordinarily designed and built so as to have the minimum size and weight necessary to accommodate the range of mechanical loads encountered in normal operation of the linkage. As a result, there is little allowance for excess loads and therefore a possibility of damage to the linkage in the event of binding or jamming at any point along the linkage. Such damage can be particularly difficult and costly to repair if it occurs in the structural support elements, since they are commonly part of larger structural members of the airplane. This is a problem even in manually actuated linkages, where large mechanical gains can result in substantial mechanical loads if jamming occurs.
For example, the rudder control system in one particular type of commercial transport airplane includes a mechanical push-pull control linkage by which the pilot controls a set of hydraulic actuators which drive the rudder. The linkage runs from pilot-operated foot pedals in the cockpit to control valve levers on the hydraulic actuators, which are located in the vertical stabilizer where they are coupled to the rudder.
Interposed in the mechanical linkage is a servocontrolled mechanical ratio changer that operates to reduce the ratio of rudder travel to pedal travel as airspeed increases. The ratio changer does this by reducing the mechanical gain of the control linkage as airspeed increases. At high airspeeds, the mechanical gain of the control linkage becomes very low, with the result that the force output capability of the ratio changer becomes very high. If, for any reason, jamming were to occur in the portion of the linkage between the ratio changer and the hydraulic actuators, with full force being applied to the rudder pedals by the pilot, the portion of the linkage between the ratio changer and the hydraulic actuators, particularly including the structural elements supporting the linkage, could be severely damaged.
The most straightforward way to avert the possibility of such damage would be to simply build the structural support components sufficiently strong to withstand the high mechanical loads that would result in the event of jamming. This approach, however, has been found to involve an unacceptably large weight penalty. In particular, most of the weight that would be added to the rudder control system by strengthening the linkage would be located in the tail of the airplane, where weight load limitations are particularly critical. Accordingly, another approach has been taken whereby it has been sought to provide a load-limiting device that may be installed as a rigid link in the mechanical linkages of the rudder control system, and which operates to limit the mechanical load on the linkage in the event of jamming or similar failure.
Various conventional approaches to this problem have been considered. For example, it has been to incorporate a shear pin assembly into the control linkage. A shear pin assembly can be incorporated into a mechanical control linkage relatively easily and with only a minimal weight addition. A primary disadvantage of shear pins, however, is that, whereas the desired shear load in many applications is on the order of two hundred pounds, the minimum shear load that can be obtained with a shear pin assembly with acceptable accuracy and repeatability is approximately 600 pounds. This is largely due to two factors. First, such shear pins are small in diameter and are therefore inherently subject to larger relative variations in strength due to the relatively larger effects of dimensional variations which are within ordinary manufacturing tolerances. Secondly, with smaller shear pins the closeness of fit between the shear pin and the pinned members is a significant factor affecting the shear strength of the assembly. More specifically, in a loosely fitted shear pin assembly there is a bending moment on the shear pin which becomes appreciable in proportion to the shear load, with resulting unpredictability in the strength of the assembly. As a result, with such small shear pins it becomes impractical to construct the shear pin assembly to the tolerances required to obtain predictable and reproducible results.
Another disadvantage of the use of shear pins is that the linkage elements connected by the shear pin assembly break free of one another in the event of shearing of the shear pin. As a result, there is a sudden and total loss of control through the mechanical linkage. Also, there is a danger of the thus disconnected linkage elements moving into a position where they may jam with one another.
Another conventional mechanical load limiter is a rotary, or cam and roller, mechanism. Such mechanisms include a spring-loaded roller engaged against a cam which forms a detent. The advantage of such a mechanism is that the cam may be readily tailored to provide a predetermined limiting force gradient. Also, the mechanism offers the capability to absorb overtravel and may be fabricated to have a negative load gradient. The primary disadvantage of such a mechanism is that it must be integrated with a crank installation and ordinarily requires a relatively large space envelope of approximately equal dimensions in width, length, and height. Also, the performance characteristic of such a mechanism cannot be readily modified once a design has been adopted, unless the change is limited to the detent, and unless such a change is of such a magnitude that it can be accomplished by revision of the cam profile.
Another previously known load limiter is a conventional linear spring cartridge. A linear spring cartridge can often be incorporated into a push-pull control rod or other mechanical linkage without the necessity of design changes to the adjoining mechanisms. However, the force gradient of a spring cartridge is invariably positive throughout its range of travel. As a result, the primary disadvantage of a spring cartridge is that a relatively long and heavy spring must ordinarily be employed to obtain an acceptably low force gradient and yet also obtain a sufficiently high initial yield force level.
Accordingly, it is the primary object and purpose of the present invention to provide an improved mechanical load limiter. More particularly, it is an object of the present invention to provide a double-acting, or bidirectional, load limiter for use in a push-pull control linkage, particularly in an airplane.
It is a further object of the present invention to provide a load limiter that operates as a rigid link in a push-pull control linkage up to a predetermined tensile or compressive load, and which operates to yield by extension or contraction in response to tensile or compression loads greater than the predetermined load. In this regard, it is yet another object of the invention to provide a load limiter that obtains the foregoing object and which yields with a substantially constant force over its entire range of extension or compression, and which also yields under the same predetermined load in both compression and tension. It is a further object to provide such a load limiter that is light in weight, compact in dimension, and which is retrofittable into preexisting control linkages.
It is also an object of the invention to provide a load limiter that is substantially free of end play in its precrushed state.
The type of failure briefly mentioned above, namely jamming of an aircraft rudder control system, is an extremely infrequent event. Such a failure might be expected to occur, on the average, no more than once in the lifetime of an entire fleet of airplanes. Accordingly, it is a further object of the present invention to provide a load limiter that is reliable over long periods of time, which requires relatively little maintenance and service over such periods, and yet which may be readily restored for use in the event it is actuated.